1. Field of the Invention
The present invention relates to a pneumatic radial tire for a heavy-duty road vehicle, and more specifically to a pneumatic radial tire for a heavy-duty road vehicle which can markedly improve durability and recapped-tire durability of the bead by suppressing bead movement during vehicle traveling.
2. Description of the Prior Art
In prior-art pneumatic radial tires for heavy-duty road vehicles such as trucks, buses, etc. as shown in FIG. 1A, a bead core B is wrapped up by a carcass K composed of metallic cords arranged at roughly 90 degrees with respect to the tire circumferential direction in such a way as to extend from the tire inner side to the tire outer side radially outwardly to a radial distance P; a chafer C of textile (i.g. nylon) or wire is arranged extending from a radial distance a along the bead wrapping area; and a rubber stock S is sandwiched between the inner portion and the outer (turn-up) portion of the carcass to firmly reinforce a bead of atire. Here, the chafer is a strip of rubberized fabric (nylon or wire) covering the bead of a tire as a protection from chafing against the rim R.
On the other hand, a strong belt layer is arranged between the carcass and a tread rubber to sufficiently increase the tire performance. The tread and the bead are connected to each other by a soft side portion of the tire mainly composed of the carcass ply and a thin sidewall rubber. Therefore, there exists a discontinuous area in rigidity between the rigid bead and the soft sidewall rubber.
As a result, when the tire sidewall is greatly deformed and the bead is greatly displaced repeatedly during tire rotation under heavy load, the rubber is peeled off near a carcass turn-up end located in the discontinuous area, thus resulting in separation faults and therefore low bead durability.
To improve the bead durability, various proposals have been made with respect to how to arrange the bead reinforcing layer such as nylon chafer, wire chafer, etc. In more detail, in the conventional way of arranging the bead reinforcing layer, since the strength of nylon chafer is relatively small, two or four nylon chafers are usually arranged. On the other hand, since the strength of wire chafer is relatively large, only a single wire chafer is usually arranged. Further, a few nylon chafers are further arranged in addition to a single wire chafer to further improve the bead reinforcement effect.
However, when the bead is reinforced by use of only nylon chafers to improve bead durability, there exists a limitation of the number of chafers because the weight is increased; the productivity is lowered; and heat is generated during traveling.
Further, when the bead is reinforced by a nylon chafer and a wire chafer in combination, there exists another problem with respect to the manufacturing equipment, in addition to an increase in weight and a decrease in productivity.
Further, when the bead is reinforced by a wire chafer, although it is possible to obtain a bead reinforcement effect to some extent without increasing the weight and decreasing the productivity, the bead is greatly moved or displaced when the tire is filled with air and a heavy load is applied to the tire, therefore, bead durability is not satisfied.
To reduce the above-mentioned bead displacement in the tire reinforced by a wire chafer, a radial distance H.sub.a between the radially inward bead base line 0 and the radially outward end a of the chafer C has conventionally been increased, as shown in FIG. 1(A), to improve the durability. In this manner of improvement, if the above radial distance H.sub.a is excessively increased, there arises another problem in that cracks will be produced in the outward end a of the wire chafer C and spreads to the tire inner liner, thus resulting in trouble.